How to create shape keys from posing armature?
If I understand your will, you should:
- 1) set the pose you want
- 2) In the armature modifier apply as shape key
- 3) in pose mode select all bones and reset all transforms (Alt G, Alt R, Alt S)
- 4) add an armature modifier to your mesh
- 5) select the mesh as "object" of the new armature modifier

In this way you will have the mesh controlled by the armature and a shape key which will reflect the pose you set in point 1)